1. Field of the Invention
This invention pertains to continuity logging and more particularly to the detection of guided waves in lithographic layers located between two well bores as a measure of continuity.
2. Description of the Prior Art
Well logging by utilizing various devices is a time honored means of recording physical measurements of the formations transversed by the borehole as a function of depth in the hole. These measurements are commonly used to identify the formations and to determine the amount and type of fluid in the formations. Such measurements are typically restricted to the region immediately adjacent to the well bore.
In addition to well-logging techniques, well bores have also been used in seismic exploration. The majority of seismic gathering procedures utilize the positioning of seismic sources and receivers on the land surface or in shallow, below-surface locations. However, when they are available, well bores have been employed in seismic exploration activity since there are some advantages afforded by having a well bore in existence. Probably the main advantage is the ability to place the source or the receiver or both closer to the reflecting geological interfaces than is possible from using surface locations alone.
When more than one borehole exist, then it is possible to utilize spaced apart boreholes for seismic exploration not possible with a single borehole. For example, U.S. Pat. No. 4,214,226, Narasimhan, et al., issued July 22, 1980, describes a procedure for producing a tomograph of the subsurface structure between boreholes by placing spaced geophones in one borehole and by producing a sequence of shots at spaced apart locations in the other borehole. The arrival times or amplitudes for transmitted primary (compressional) waves which travel directly between shot and geophone pairs are used to produce an image of the structure between the boreholes. In another example, U.S. Pat. No. 4,298,967, Hawkins, issued Nov. 3, 1981 a procedure for providing a horizontal profile of a reflector located between boreholes by using multiple shot and receiver positions in the boreholes. In this case, the arrival times from reflected primary waves which travel from the source to the reflector and then to the geophones are used to image the reflector.
Another type of subsurface seismic exploration involves placing sources and receivers within a coal seam and along roadways in mines. In this application, guided waves, which are trapped within the coal, and which are typically called seam waves or channel waves, are used to determine the location of faults in the seam. Unlike body waves such as primary and secondary (compressional and shear) waves, guided waves are confined in space. These guided waves are trapped in low-velocity layers such as coal by critical reflections of the wave at the upper and lower boundaries of the layers. Because these guided waves have their largest amplitudes in the coal, they can be used to investigate discontinuities in the coal. Guided waves are often used to measure discontinities in three-foot coal seams even though the wavelength of the body waves generated from the same source may be 100 feet long. The resolution of seismic imaging using body waves is limited to one half of a wavelength which is often much larger than the coal seam.
A number of techniques have been developed to enhance the detection of guided waves in coal seams. These techniques require the detectors to be deployed in a mine at specific location within the coal layer. For example, U.S. Pat. No. 3,352,375, Krey, issued Nov. 14, 1976, uses geophone pairs with one geophone located near the upper boundary of the seam and the other located near the lower boundary of the seam. The signals from the two geophones in each pair are subtracted. In another patent, U.S. Pat. No. 3,858,167, Stas, et al., issued Dec. 31, 1974; the geophones are located in the center of the seam and oriented in a specific direction to provide maximum sensitivity to the seam waves. The technique described in U.S. Pat. No. 4,351,035, Buchanan, et al., issued Sept. 21, 1982, requires an array of geophones located along a roadway to generate a holograph of the seam.
The development of a procedure for ascertaining the existence of continuity in one or more layers as opposed to discontinuity existing in other layers can be extremely useful for both exploration and production of oil and gas. In exploration, for example, continuity logging can be useful for constructing geologic models, correlating formation properties between boreholes, and determining the presence of faults. In production, continuity logging can be used to indicate the continuity of the reservoir between the boreholes and the presence of permeability barriers. In addition, continuity logging can be useful in planning the mining of coals and other minerals.
Therefore, it is a feature of the present invention to provide an improved method of detecting the presence of a continuous layer between boreholes in the subterranean lithography therebetween.
It is another feature of the present invention to provide an improved method of detecting the presence of guided waves in specific lithographic layers as a means of determining the continuity of such layers.